The production of amino acids in accordance with a transamination reaction involves the covalent transfer of an amino group from an amine donor to an acceptor alpha-keto-acid precursor of the amino acid in the presence of an aminotransferase. A transamination is illustrated by the following general reaction:
wherein (I) is an amine donor; (II) is a keto acid precursor, (III) is a keto acid product and (IV) is an amino acid and a.t. is an aminotransferase. The reaction is reversible, thus the aminotransferase may be utilized both in the synthesis and the degradation of an amino acid.

Amino acid synthesis often involves transamination as the final step in biosynthesis. For example, the biosynthesis of the aromatic amino acids phenylalanine and tyrosine requires the action of an aminotransferase as the final step. Phenylpyruvic acid is converted to phenylalanine by an aminotransferase that transfers an amino group from glutamate. The keto-acid product of this reaction isα-ketoglutaric acid. Similarly, an amino group from glutamate is transferred to 4-hydroxyphenylpyruvic acid to produce tyrosine.

The aminotransferases employed in the transamination reactions may be obtained from a variety of sources, in particular, from microorganisms producing the aminotransferase. Naturally occurring microorganisms, so-called "wild-type" microorganisms, are capable of producing a variety of aminotransferases.

In general, the rate of amino acid production in accordance with a transamination reaction is determined by the activity of the particular aminotransferase employed and the temperature at which the transamination reaction is conducted. Most aminotransferases produced by wild-type microorganisms lose significant activity at temperatures equal to or greater than 40̸°C. In addition, the aminotransferases produced by wild-type microorganisms lose activity in the presence of high concentrations of a keto acid precursor, and/or amino acid product.

It is an object of the present invention to provide a method to increase the rate of amino acid production in accordance with a transamination reaction by reducing the effects of the loss of aminotransferase activity due to temperature and to enable one to conduct a transamination reaction using high concentrations of keto acid precursor.

In addition, it is an object of the present invention to provide a method for producing amino acids in accordance with transamination reaction using an aminotransferase obtained from a genetically modified microorganism capable of reproducing the aminotransferase wherein the microorganism is made permeable during the transamination reaction.

It is a further object of the present invention to provide a process for producing amino acids in accordance with a transamination reaction conducted at a temperature greater than the temperature at which the aminotransferase will ordinarily limit the usefulness of the process.

The present invention provides a process for preparing an amino acid by transamination between an amine donor and keto acid precursor of the amino acid in the presence of an aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase wherein the transamination is carried out at a temperature equal to or greater than 40°C.

The present invention also provides a process for preparing phenylalanine by transamination between an amine donor and phenylpyruvic acid in the presence of an aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase, wherein the keto acid precursor concentration is greater than 10g/L.

In accordance with the process of the present invention, high yields of an amino acid are obtained at a high rate of production utilizing a transamination reaction employing a genetically modified microorganism that overproduces the aminotransferase. The term "genetically modified microorganism that overproduces the aminotransferase" refers to a genetically modified microorganism that produces the aminotransferase at a level of at least 200U/g dry weight cells. As used herein, the term "rate of conversion" refers to the "grams of amino acid produced/gram dry weight of cells (or microorganisms) present X time to equilibrium" or to the grams of amino acid produced/10⁴ Units of enqyme activity X time to equilibrium. As used herein, the term "Units of enzyme activity" refers to the enzymatic activity of the reaction mixture produced by the microorganisms present in the reaction mixture. The term "time to equilibrium" refers to the time at which the maximum concentration of amino acid is produced by the reaction.

In accordance with a preferred embodiment of the present invention a microorganism capable of overproducing aminotransferase of interest is grown in suitable media. The microorganism generally contains a plasmid carrying a gene encoding an aminotransferase capable of transaminating a keto acid precursor of the amino acid to be produced. Such media are well recognized by one of ordinary skill in the art and include for example, a media comprising yeast extract, glutamate and mineral salts. A preferred media comprises glucose and mineral salts. Such media are generally commercially available or readily prepared by one of ordinary skilled in the art. The cells are cultivated within a temperature range of from 25° to 40̸°C and preferably about 37°C for approximately sixteen hours or until the concentration of cells is preferably at least 10̸g/L and most preferably 30̸g/L. The cells may be utilized in the presence of the growth media or harvested by centrifugation. In addition the cells may be utilized intact or may be treated to release the aminotransferase. Techniques for releasing the aminotransferase are well known in the art and include, for example, treatment of the cell with a surfactant, e.g., 0.2% Triton X-10̸0̸ 0̸.2% deoxycholate etc.; physical breakage via freeze/thawing; sonication; pressure disruption and the like. It has been surprisingly found that if intact cells are utlized in accordance with the procedure of the present invention, the cells are made permeable at temperatures at which the transamination reaction is conducted.
Specifically, it has been found that when the process of the present invention is conducted at temperatures greater than 40̸°C, the cells are permeabilized. In an embodiment of the present invention it is preferred not to permeabilize the cells prior to the transamination.

Therefore intact cells may be added to a mixture containing the amine donor and keto acid precursor. If the cells are permeabilized prior to conducting the transamination, a portion of the solution containing the released aminotransferase is added to a mixture containing an amine donor and a keto acid precursor. Preferably the resulting reaction mixture is incubated for a period of time sufficient to allow the reaction mixture to reach equilibrium.

Depending on the specific aminotransferase employed, the pH of the reaction-mixture is maintained within a range of from 5 to 9. In the production of phenylalanine, the reaction mixture is preferably maintained within a range of from 6.5 to 9 and more preferably of from 7.5 to 8.5. Suitable buffers may be used to maintain the pH of the reaction mixture within a range sufficient to maintain enzyme activity.

In order to increase the yield of amino acid and stabilize the reaction mixture, a cofactor to the aminotransferase such as pyridoxal-5-phosphate, pyridoxine, pyridoxamine or derivatives thereof may be added to the reaction mixture. The quantity of cofactor to be added to the reaction mixture is readily ascertained by one of ordinary skill in the art.

Furthermore, the addition of a formic acid solution to the reaction mixture may increase the rate of reaction and production of the amino acid. It is preferred to employ a formic acid concentration in the range of from about 0̸.5M to about 15M. The addition of formic acid to the reaction mixture may increase the rate of reaction by increasing the activity of the aminotransferase and stabilizing the aminotransferase particularly if the reaction is conducted at temperatures equal to or greater than 40̸°C.

Representative amino acids produced in accordance with the methods of the present invention include, but is not limited to, L-arginine, L-glutamine, L-serine, L-tyrosine, L-phenylalanine, L-tryptophane, L-dihydroxyphenylalanine (L-DOPA), L-histidine, L-lysine, L-isoleucine, L-methionine and the like. The methods of the present invention are particularly effective in producing aromatic amino acids such as for example L-phenylalanine, L-tyrosine, and the like.

The selection of the specific amine donors and keto acid precursors employed in the process of the present invention is dependent upon the specific amino acid to be produced and is readily ascertained by one of ordinary skill in the art. A wide variety of amine donors may be employed depending upon the aminotransferase utilized. If aspartate aminotransaminase is employed as the aminotransferase, glutamic acid, alanine, aspartic acid, tyrosine or derivatives thereof, may be employed as amine donors. The keto acid precursor employed depends upon the specific amino acid to be prepared. The keto acid precusor may be utilized in the form of the free acid or salt thereof. For example, in the production of phenylalanine, phenylpyruvic acid or derivatives thereof, is employed as the keto acid precursor. The following table illustrates suitable keto acid precursors that may be employed to produce various amino acids:

The ratio of amine donor to keto acid precursor may be readily ascertained by one of ordinary skill in the art. Although, the theoretical ratio of amine donor to keto acid precursor should be at least 1:1, it is preferred to employ an excess of the amine donor. For example, in the production of phenylalanine, it has been found that if glutamic acid is employed as the amine donor, a ratio of glutamic acid:phenylpyruvic acid in the range of from about 3:1 to 4:1 is preferred. Furthermore, it has been found that if aspartic acid is employed as the amine donor in the production of phenylalanine, it is preferred to employ a ratio of aspartic acid: phenylpyruvic acid in the range of from about 1:1 to 1.5:1 and most preferred in a range of 1:1 to 1.2:1.

It has been surprisingly found that keto acid precursor concentrations greater than 10̸g/L have been effective in the process of the present invention. In the production of L-phenylalanine, it has been found that concentrations of phenylpyruvic acid in the range of 30̸-60̸g/L are preferred. In addition it has been found that reaction by-products produced in the preparation of phenylpyruvic acid and generally transferred to the transamination reaction mixture when phenylpyruvic acid is employed as the keto acid precursor, (i.e., salts, dimers, etc.) do not affect the yields of phenylalanine. Furthermore, it has been found that the phenylpyruvic acid employed as the keto acid precursor may be commercially purchased or prepared in situ and employed without isolation or purification in the transamination reaction of the present invention. Phenylpyruvic acid may be prepared by reacting benzaldehyde and hydantoin to yield benzylidenehydantoin and then hydrolyzing the benzylidenehydantoin to phenylpyruvic acid or salts thereof.

In addition to temperature and keto acid precursor concentration, the aminotransferase activity of a reaction mixture depends upon the amount of cells containing the aminotransferase added to the reaction mixture. The lower the aminotransferase activity of each individual cell, the more cells that must be added to the reaction mixture to sustain the reaction. However, the greater the amount of cells added to the reaction mixture, the greater the difficulty in isolating the amino acid product. One advantage associated with the process of the present invention in employing a genetically modified microorganism in lieu of a wild type microorganism is that it has been found that it is possible to maximize the aminotransferase activity in the reaction mixture and minimize the total amount of cells present, thereby facilitating product isolation. The low cell density of the reaction mixture in the process of the present invention enhances amino acid product recovery by facilitating the removal of cell debris. It has been found that a concentration of cells sufficient to yield an aminotransferase activity of o.5 x 10̸⁴ U/L or greater is preferred when conducting the process of the present invention.

The amino acids produced in accordance with the methods of the present invention may be isolated in accordance with conventional techniques. For example, a water miscible solvent wherein the amino acid of interest is insoluble may be added to the reaction mixture. Upon addition of the water miscible solvent, the amino acid of interest will precipitate and thus be removed from the reaction mixture. This method for isolating the amino acid product may be of particular importance with respect to amino acids such as phenylalanine, that inhibit the aminotransferase employed in the transamination reaction.

In accordance with the process of the present invention, a wide variety of aminotransferases produced by a microorganism may be effective in the procedures of the present invention. Examples of such aminotransferases include but are not limited to aminotransferases such as aspartate aminotransferases, branched chain aminotransferases and aromatic aminotransferases as defined by the gene products E.coli genes aspC, tyrB and ilyE and the like. The above-mentioned aminotransferases when produced using a genetically modified microorganism are not necessarily pure. However, it has been found that unpurified crude extracts containing the aminotransferase of interest may be employed in the process of the present invention. These extracts may be of course, partially purified prior to being utilized in accordance with the methods of the present invention. A mixture of two or more aminotransferases may also be employed in accordance with the methods of the present invention. In the preparation of phenylalanine in accordance with the methods of the present invention, aspartate aminotransferase is the preferred aminotransferase employed.

In view of the fact that production of amino acids by conventional transamination reactions is limited by aminotransferase activity and that increasing the temperature of the reaction substantially reduces and/or effectively inhibits aminotransferase activity, it has been found that in order to produce amino acids in accordance with a transamination reaction at temperatures equal to or greater than 40̸°C, it is necessary to increase the aminotransferase activity to a level sufficiently high to overcome effects of denaturation of the aminotransferase due to temperature and/or high keto acid concentration. Because a wild type microorganism will not produce aminotransferases in quantities sufficient to conduct the processes of the present invention, it is necessary to employ a genetically modified microorganism which will over-produce the aminotransferase. As readily ascertained by one of ordinary skill in the art, microorganisms may be readily genetically modified in accordance with conventional techniques. Such techniques include, for example, mutagenesis, transduction, transformation, (e.g. with multi copy plasmids), conjugation and in vitro gene manipulation. For example, aminotransferase over-producing strains may be isolated after mutagenesis. Alternatively, the gene for the appropriate aminotransferase may be cloned into a suitable plasmid or phage vector. A further embodiment invokes isolation of the aminotransferase gene on a suitable vector and the subsequent in vitro or in vivo manipulation so as to elevate expression of the aminotransferase gene. This may be accomplished for example, by fusing the gene to a more powerful constitutive promoter. Then, the isolated, mobilized aminotransferase gene may be inserted back into the microorganism chromosome by techniques such as recombination, transposition or phage interactions.

The microorganisms which may be genetically modified as described herein include but are not limited to, for example, Achromobacter aquatilis, Achromobacter liquidum, Aspergillus oryzae, Aspergillus niger, Bacillus species such as Bacillus megaterium, Bacillus subtilis, Bacterium succinium, Brevibacterium species such as Brevibacterium flavum, coryneforms including Corynebacterium species such as Corynebacterium glutaminicum, Erwinia herbicola, Escherichia coli, Gluconobacter melanogenus, Lactobacillus bulgaris, Micrococcus ureae, Penicillium vinaceum, Proteus vulgaris, Pseudomonas species such as Pseudomonas aeruginosa, Pseudomonas dacunhae, Pseudomonas putida, Sarcina lutea, Streptomyces griseus, Serratia marcescens, Streptomyces phaeochromogenus, and Saccharomyces species and Schizosaccharomyces species. These microorganisms may not necessarily be intact living cells, but may be lyophilized, heat-treated or treated with acetone and the like to permeabilize the cells prior to use thereof in the present invention.

For the purpose of illustrating the processes of the present invention, a strain rich in aminotransferase may be constructed by cloning the gene for the aspartate aminotransferase from E-coli K12 (aspC) into the multi-copy plasmid pAT153. The cloned strain is transformed into a suitable strain such as E.coli K12 W3110̸, and the resulting strain over-produces the aspC gene product from 10̸-10̸0̸ times. The aspartate aminotransferase is known to be active in the transaminations; i.e., the conversion of phenylpyruvic acid to phenylalanine; using both L-aspartic acid and L-glutamic acid as amine donors. The aspC encoded protein has the additional benefit of being relatively heat stable.

An aspC clone may preferably be obtained by ligating suitably restricted DNA from either E.coli K12 or another suitable microorganism into a suitable vector such as pAT153. The resulting ligation reaction is then used to transform a suitable recipient such as HW159 (ATCC 39260̸) deposited in American Type Culture Collection, 1230̸1 Parklawn Drive, Rockville, Maryland 20̸852 on 6 January 1983 under Budapest Treaty Conditions. This strain (HW159) lacks both the aspartate aminotransferase (aspC) and aromatic aminotransferase (tyrB) genes. The result of these two lesions is that the strain requires tyrosine, aspartate and to a lesser extent, phenylalanine for growth. Recombinants that carry aspC or tyrB clones may be readily identified by their ability to grow on minimal medium in the absence of tyrosine, aspartate and phenylalanine. AspC clones may be differentiated from tyrB clones by analysis of cells extracts by native polyacrylamide gel electrophoresis followed by staining for phenylpyruvic acid aminotransferase activity. Isolation of an aspC clone may be further facilitated by the use of a specialised transducing phage such as Lambda aspC as a source of DNA in the original ligation.

The microorganism (ATCC 3950̸1) used in the following examples was constructed by transforming an E.coli K12 prototroph with the plasmid pME98, a pAT153 derivative carrying the aspC gene on a 3.4kb fragment of DNA. ATCC 39501 was deposited in the American Type Culture Collection on 4th November 1983 under Budapest Treaty conditions. All the techniques are conventional techniques readily ascertained by one of ordinary skill in the art.

One embodiment of the present invention relates to an improved process for preparing an amino acid in accordance with a transamination reaction involving an amine donor and keto acid precursor of the amino acid in the presence of an aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase, wherein the improvement comprises conducting the transamination reaction at a temperature equal to or greater than 40°C. In the production of phenylalanine in accordance with the process of the present invention, it is preferred to conduct the transamination reaction within a temperature range of from 40°C - 70°C and most preferred to conduct the reaction within a temperature range of 45°C - 60°C.

Another embodiment of the present invention relates to an improved process for preparing phenylalanine in accordance with a transamination reaction involving an amine donor and keto acid precursor of phenylalanine in the presence of an aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase, wherein the improvement comprises having a keto acid precursor concentration of greater than 10 g/L. It is preferred to have a keto acid precursor concentration in a range of from 30 to 50 g/L and preferably from 40 to 50 g/L and to conduct the transamination reaction within a temperature range of 45° - 70°C, particularly from 45°C to 55°C.

Another embodiment of the present invention relates to an improved process for preparing an amino acid in accordance with a transamination reaction involving an amine donor and keto acid precursor of the amino acid in the presence of aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase wherein the improvement comprises permeabilizing the microorganism during the course of the transamination reaction by conducting the reaction at a temperature equal to or greater than 40̸°C.

A further preferred embodiment involves the production of aromatic amino acids in accordance with a transamination reaction employing an aminotransferase obtained from a microorganism which has been genetically modified to overproduce the aminotransferase, wherein the reaction is conducted at a temperature equal to or greater than 40̸°C.

The following Examples are intended to further illustrate the present invention and not to limit the invention in spirit or scope. In the Examples, all parts are parts by weight unless otherwise expressly set forth. The term "cells" as used herein in the following Examples refers to genetically modified cells containing elevated levels of the aminotransferase required to conduct the transamination reaction.

Escherichia coli containing a plasmid carrying a gene encoding for aspartate aminotransferase was grown in one of the following media: - 1) Yeast extract (10 g.), Na₂HPO₄ (6 g.), KH₂PO₄ (3 g.), NaCl (0̸.5 g.), NH₄Cl (1 g.), CaCl₂·6H₂O (0̸.0̸2 g.), MgSO₄·7H₂O (0̸.25 g.), Carbenicillin (0̸.1 g.) and water to 1 litre; 2) glucose (35g), KHPO₄ (6.6g), (NH₄)₂HPO₄ (2.0̸g), MgSO₄.7H₂O (2.25g), ammonium ferric citrate (0̸.11g), indole acrylic acid (2.0̸mg) and water to 1 liter. The cells were cultivated at 33°C for a period of time sufficient to ensure at least 1 g dry weight of cells per litre. The cells were harvested by centrifugation and the aspartate aminotransferase was released by freezing and thawing the resulting cells suspension containing approximately 1 g dry weight of cells per liter.

A solution containing aspartate aminotransferase was produced as in Example 1 from a suspension containing approximately 2 g dry weight cells per liter. The solution was added to a mixture comprising 15 g sodium phenylpyruvate monohydrate per liter, 19 g sodium aspartate monohydrate per liter, 10̸ mg pyridoxal phosphate per liter adjusted to pH 8.5 with 10̸N sodium hydroxide. The resulting reaction mixture was incubated for 4 hours at 60̸°C to yield a mixture containing 10̸.2 g L-phenylalanine per liter.

A flask was shaken at 50̸°C with the following reaction mixture: 3.88g aspartic acid, 5ml cell suspension containing 0̸.14 X 10̸⁴U aspartate aminotransferase, 0̸.375mg pyridoxal phosphate and 51ml of a reaction mixture prepared by heating under reflux, with stirring, for 3.5 hours, 4-hydroxy-5-benzylidene hydantoin (8.50̸g/41.67mmole) sodium hydroxide pellets (48.0̸g/1.20̸mole) and water (240̸ml). The reaction mixture was incubated overnight at 50̸°C and the resulting precipitate was filtered, washed and identified as L-tyrosine by NMR, TLC, HPLC and optical rotation ([α]^20̸ = - 10̸.33° (C = 4, 1N HCl))

A series of transamination reactions were conducted under various conditions in accordance with the procedure of Example 2. The temperature at which the reaction is conducted, the initial phenylpyruvic acid concentration ([PPA]), amine donor, time to equilibrium, yields of phenylalanine ([L-Phe]), grams of cells % Conversion and rate of production of phenylalanine (g phenylalanine/g cells x time to equilibrium) (Rate) is represented in the following table:

A series of transamination reactions were conducted under various conditions in accordance with the procedure of Example 2. The temperature at which the reaction is conducted, the initial phenylpyruvic acid concentration ([PPA]), amine donor, yields of phenylalanine ([L-Phe]), % Conversion and Units enzyme activity ("Units") and rate of production of phenylalanine (g phenylalanine/ Units of enzyme activity x time to equilibrium) (Rate) is represented in the following table:

A series of transaminations were conducted in accordance with the procedure of Example 2 at a temperature of 50̸°C using 40̸ g phenylpyruvic acid per liter, 48.6g/L of aspartic acid and varying amounts of cells in order to vary the aminotransferase activity. The results in Table 4 illustrate the aminotransferase activity, grams of cells employed, and the time needed for the reaction to reach equilibrium.

A reaction vessel containing a 9.6L of a mixture comprising 585g sodium phenylpyruvate monohydrate and 324g aspartic acid was heated to 50̸°C and the pH of the mixture was controlled at pH 8.0̸ by the addition of either 35% NH₄OH or 18% HCl as appropriate. To the reaction mixture was added 40̸0̸ml of an homogenized cell suspension containing 3.4 x 10̸⁵ U aspartate aminotransferase and 75mg pyridoxal phosphate. The resulting mixture was stirred for 3.5 hours to yield a solution containing 33.9g phenylalanine/L, at a rate of 7.1g L-phenylalanine/10̸⁴ Units.hr.

A reaction vessel containing a 9.7L of a mixture comprising 585g sodium phenylpyruvate monohydrate and 486g aspartic acid was heated to 50̸°C and the pH of the mixture was controlled at pH 8.0̸ by the addition of 35% NH₄OH. To the reaction mixture was added 275ml homogenised cell suspension containing 2.8 x 10̸⁴ U aspartate aminotransferase and 75mg pyridoxal phosphate. The resulting mixture was stirred for 3.6 hours to yield a solution containing 36.2 g phenylalanine/L, at a rate of 3.6g L-phenylalanine/10̸⁴ Units.hr.

A flask was shaken at 50̸°C with the following reaction mixture made up to 50̸ml with water - 1.0̸5g 3-methoxy-4-hydroxyphenylpyruvic acid, 1.30̸g sodium aspartate monohydrate, 0̸.375 pyridoxal phosphate and 5ml homogenised cell suspension. The pH of the reaction mixture was adjusted to 8.0̸ with 1N NaOH. The resulting reaction mixture was incubated at 50̸°C for 2.5 hours to yield or mixture containing 11.3g/L of 3-methoxy-4-hydroxyphenylalanine.

A reaction mixture containing 2.49g sodium phenylpyruvate monohydrate, 3.16g sodium aspartate monohydrate 0̸.375mg pyridoxal phosphate and 2.5ml intact cells containing 0̸.2 x 10̸⁴U aminotransferase and 50̸ml water was adjusted to pH 8.0̸ with IN NaOH.

The reaction mixture was incubated for 2 hours at 50̸°C to yield 32.0̸g of L-phenylalanine per liter.

Although this invention has been described with respect to specific modification, the details thereof are not to be construed as limitations, for it will be apparent that various equivalents, changes and modification may be resorted to without departing from the spirit and scope thereof and it is understood that such equivalent embodiments are intended to be included therein.